1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a supply invariant bandgap reference system.
2. Description of the Related Art
Electronic systems represent a wide range of systems including controllers for switching power converters, microprocessors, and memories. Electronic systems include digital, analog, and/or mixed digital and analog circuits. The circuits are often implemented using discrete, integrated, or a combination of discrete and integrated components. To properly operate, many electronic systems utilize one or more voltage and/or current reference generators. In many instances, particularly for analog circuits, more precise circuits utilize more precise reference signals. Thus, in many instances, the reference generators attempt to provide a stable reference signal over variations in supply voltage and temperatures. A bandgap reference represents an accepted choice to supply the reference signal. In general, bandgap references refer to the utilization of a voltage difference between two p-n-junctions operating at different current densities to generate the reference signal.
FIG. 1 depicts a bandgap reference 100, which provides a bandgap reference voltage VBG. In general, the bandgap reference 100 develops the bandgap reference voltage VBG based on the inherent forward-biased voltages across diodes 102 and 104. The bandgap reference 100 receives power from a voltage source having a voltage VCC referenced to a ground reference 101. When forward biased, diodes 102 and 104 have respective forward biased voltages VBE1 and VBE2. Voltage VBE2 is a fraction of voltage VBE1. A desired ratio of voltages VBE2 to VBE1 can be achieved by increasing the size, and, thus, the current density, of diode 104 relative to diode 102 or placing multiple diodes in parallel to collectively from diode 104. Operational amplifier 106 maintains the voltage VNN equal to voltage VNP by driving the gate of p-channel metal oxide semiconductor field effect transistor (PMOSFET) 112 in accordance with the difference voltage of VNN-VNP. For VNN>VNP, current iC2 decreases, and for VNN<VNP, current iC2 increases. The voltage VNP is at the cathode of diode D1. Accordingly, the bandgap reference voltage VBG is derived as follows with “R” being the resistance value of resistors 110 and 111 and “R1” representing the resistance value of resistor 108:VBE2+iC2·R1=VBE1  [1];iC2·R1=VBE1−VBE2=ΔVBE  [2];Since VNN=VNP,iC1=iC2,then iC1·=ΔVBE/R1  [3];iC1·R=VNN−VBG=(ΔVBE·R)/R1  [4]; andVBG=VBE1+(ΔVBE·R)/R1  [5].
In at least one embodiment, bulk error currents develop in semiconductor bulk material, especially with changes and increases in the supply voltage VCC. Bulk error currents occur because of, for example, hot electron injection of current in a semiconductor device, such as a metal oxide semiconductor field effect transistor (MOSFET). The bulk error current occurs when, for example, “hot” electrons cross an energy barrier in a channel region of the MOSFET. In a stable environment with an approximately constant bulk error current iBULK—ERROR, bandgap reference 100 provides a relatively stable bandgap reference voltage VBG. However, in some environments the direct current (DC) component of supply voltage VCC varies by 100-200% or more, e.g. 6V<VCC<18V, and alternating current (AC) signals, such as transient voltages and ripples, in supply voltage VCC can cause high frequency variations in supply voltage VCC. Variations in the supply voltage VCC tend to vary and, thus, destabilize the bulk error current iBULK—ERROR. Variations in the bulk error current iBULK—ERROR destabilize the currents iC1 and iC2 and, thus, cause the bandgap reference voltage VBG to vary. Variations of the bandgap reference voltage VBG can cause errors in circuits, such as analog-to-digital converters, that rely upon a stable bandgap reference voltage VBG to function properly and accurately.